The objectives of this study are a) to determine the importance of extravascular forces or tethering to venous shape and function, b) to correlate vein wall composition to vein properties, c) to determine the long term role of pressure in determining wall composition and properties, and d) to determine the involvement of large veins in the carotid sinus reflex. Tethering will be studied by measuring diameter and resistance as a function of pressure in unexposed and exposed veins. Diameter will be measured using radiographic visualization. Resistance will be determined from pressure gradients across the segment at known flow rates. A comparison of the diameter and resistance data will indicate if the vein is circular. Vein shape will also be determined by x-raying in situ veins from 3 different angles. Diameter and resistance will be measured as a function of pressure relaxed and activated. The amounts of connective tissue, electrolytes, and smooth muscle will be determined using standard chemical techniques and correlated to anatomical variations in mechanical properties. Variations in mechanical properties and composition will also be correlated to pressures measured in the erect dog. The long term role of pressure will be studied by surgically creating an arterialvenous fistula in one hindlimb and leaving the other normal. Venous pressures in both limbs will be monitored for 30 to 60 days. After this period a comparison of the mechanical properties and composition in the veins from both limbs will show if a chronic elevation in pressure results in changes in vein wall structure and function. The amount of vein constriction resulting from changes in carotid sinus pressure will be radiographically determined for 5 anatomically different veins. In each animal one carotid sinus will be perfused at 3 different pressures, the other will be denervated.